I do a lot of diluting. I HATE most low calorie salad dressings, so I'll often dilute a full fat dressing with rice wine vinegar to make the dressing go further. Rice wine vinegar is so mild I can make a nearly oil free dressing with it, or dilute a ful fat dressing.
I dilute barbecue sauce with diet cola
When I was eating cereal, I'd add a 1/4 cup of a sweetened cereal to a bowl of very low calorie dry cereal.
Not really a dilution, but thinking of cereal made me think of a dessert I really like which is hot flavored oatmeal with a scoop of no sugar added vanilla ice cream. So yummy, almost like a cobbler (adding a tablespoon or two of melted low cal jam or fruit spread, once in a while is extra special).
I brown ground beef with dry tvp (soy protein, looks kind of like grape nuts cereal, often sold in bins at most health food stores). The tvp is only about $2.50 per pound, and each pound is equivalent to about 3 lbs of hamburger. It has to be reconstituted with an equal volume of water, so I'll brown 1 cup of tvp (equivalent to about a lb of hamburger) with 1 lb of ground beef and I'll let them brown together for a few minutes and then add 1 cup of hot water or canned beef broth. Sometimes I brown it with onions, celery, bell pepper. I freeze it in ziploc bags or tupperware and as it freezes, I shake and break up the chunks every 20 minutes or so, so it freezes in crumbles. Then I just scoop out what I need for recipes that start with ground beef.
TVP can be used as a ground beef substitute, but it doesn't have much flavor of it's own. I don't mind using tvp instead of ground beef, but hubby wants "real beef" flavor, so I started combining the two, and I've converted hubby, because he will even remind me in the store if we're running low on tvp. We're on a fairly tight budget, so anything that not only saves calorie, but saves money is pretty darned special.
I use beans to extend foods also, some of which normally wouldn't contain beans. I will cook dry beans and freeze them in ziploc or tupperware, and do the shake and break so they're scoopable frozen also. For example, mild beans like pink beans or most white beans are a great extender for spaghetti. We had a last minute dinner guest one night, my hubby's friend who is now our neighbor in our apartment building. I had already made the spaghetti and beans (I do try to go a bit more traditional, if we're having expected guests), and he looked at it rather suspiciously but he sure dug in once he'd tasted it. He's now on my list of approved guinea pigs for new dishes.